The 4.0 is known as a reliable but noisy engine. Every 4.0 I've owned has had some kind of tick to it, some being louder then others. I've tried synthetic, conventional, 5w30, 10w30, 5w40, 10w40 and some of those mixed with Marvel Mystery Oil without much luck. The Marvel Mystery made the biggest difference out of the above, but still not much.

Over on the Bob Is The Oil Guy Forums I got into a discussion about 4.0 valvetrain noise. A member of the forum suggested I substitute about 1/2 quart of 5w20 synthetic with whatever oil I was running. I filled the Jeep with 5.5 quarts of Valvoline MaxLife 10w40 and 1/2 quart Pennzoil Platinum 5w20 synthetic. The oil pressure is perfect and the jeep sounds much quieter. The valvetrain noise is half of what it was before.

The belief is that the smaller molecules of the 5w20 better lubricate the hydraulic lifters, similar to what the Marvel Mystery did which is why that slightly lessened the noise.

Lifters are down low in the side of the block. Valve train is up high on top. Where is the noise?

Both can have noise. Mine has some lifter noise and valvetrain noise. The 5w20 helped with both. I can hardly hear the lifters and the valvetrain is quieter. This was verified with a mechanics stethoscope.

When I was first told about this I kinda laughed. 5w20 is for Hondas, in fact its what my mom's '05 Civic is spec'd for. It does work though to my surprise. Same NAPA Gold 1085 oil filter with every change for those who are wondering.

I've always used Synthetics in my Jeeps but the engine is still noisier than any other vehicle I've owned.
Quieting the engine will only make all the other extraneous Jeep sounds more noticeable...LOL
Jeeps are noisy...Live with it

I have a 1998 Jeep Grand Cherokee Larado w/ 6 cy engine. I have noticed over the full 4-5 years that I have owned it that there is a bit of a jump when the tramsmission initially shifts. It has never changed and has never gotten worse. Can anyone tell me what my problem really is?? lb1047

I've always used Synthetics in my Jeeps but the engine is still noisier than any other vehicle I've owned.
Quieting the engine will only make all the other extraneous Jeep sounds more noticeable...LOL
Jeeps are noisy...Live with it

Kinda missing the point... I said that I've used synthetics. I've used synthetic 5w30, 10w30, 5w40, etc. The little bit of 5w20 synthetic mixed with the MaxLife seems to help. For what its worth my jeep was louder with only synthetic in the crankcase.

AZJeff:

Piston slap is usually most noticeable on startup, when the engine warms up the heat causes them to expand and get quieter. My noise seemed more noticeable once the engine warmed up. Piston slap is more likely on '91-'95 4.0L's. In '96+ they changed to aluminum pistons. This didn't completely eliminate the issue, but helped.

I'm only suggesting this since it worked for me and others. Obviously every jeep is different and this won't eliminate all noise from the motor. This is just a suggestion.

Piston slap is usually most noticeable on startup, when the engine warms up the heat causes them to expand and get quieter. My noise seemed more noticeable once the engine warmed up. Piston slap is more likely on '91-'95 4.0L's. In '96+ they changed to aluminum pistons. This didn't completely eliminate the issue, but helped.

They always had aluminum pistons... nobody has used anything for pistons except aluminum since the 1940s. What they did in the later 4.0 years was use hypereutectic (high-silicon) pistons with coated skirts instead of bare skirts, which allows a tighter fit and then the coating (PTFE -based) "wears to fit" without scuffing the cylinder walls.

As for the quieting effect of 5w20... I don't really buy it as explained. Mixing a 5w20 oil with a thicker oil results in an oil that, overall, is pretty much like a 10w30. It won't be any quieter (or louder) than straight 10w30. The thin oil doesn't somehow have "smaller molecules," it just has a different thickness, which isn't necessarily related to molecule size but can be related to things like viscosity index improvers. Maybe your engine just "likes" a thinner oil overall, have you tried something like one of the light 30-grades, say Mobil 1 0w30AFE?

For what its worth, I find both of my 4.0s to be plenty quiet on Rotella T6 5w40, but then they are later models (1999 and 2001) which have the coated pistons, main bearing girdle, and other things that the early 4.0 didn't have. I have never thought the 4.0 is inherently any louder than any other engine- it just doesn't have any added sound deadening like multi-layered composite valve covers and oil pans, which other modern engines have and which do a very good job of keeping the engine noise contained inside the engine.

They always had aluminum pistons... nobody has used anything for pistons except aluminum since the 1940s. What they did in the later 4.0 years was use hypereutectic (high-silicon) pistons with coated skirts instead of bare skirts, which allows a tighter fit and then the coating (PTFE -based) "wears to fit" without scuffing the cylinder walls.

As for the quieting effect of 5w20... I don't really buy it as explained. Mixing a 5w20 oil with a thicker oil results in an oil that, overall, is pretty much like a 10w30. It won't be any quieter (or louder) than straight 10w30. The thin oil doesn't somehow have "smaller molecules," it just has a different thickness, which isn't necessarily related to molecule size but can be related to things like viscosity index improvers. Maybe your engine just "likes" a thinner oil overall, have you tried something like one of the light 30-grades, say Mobil 1 0w30AFE?

For what its worth, I find both of my 4.0s to be plenty quiet on Rotella T6 5w40, but then they are later models (1999 and 2001) which have the coated pistons, main bearing girdle, and other things that the early 4.0 didn't have. I have never thought the 4.0 is inherently any louder than any other engine- it just doesn't have any added sound deadening like multi-layered composite valve covers and oil pans, which other modern engines have and which do a very good job of keeping the engine noise contained inside the engine.

Thanks for explaining the pistons. I went back and check my sources and you are correct. I've used Rotella T 5w40 and didn't notice much of a difference. I thought the same thing about the 5w20 and 10w40 basically making a 10w30. However, the 10w30 I've used seems louder.

Instead of mixing a quart of light synthetic oil with regular dino oil, why not go for a 5w oil ... wouldn't that be just as "thin" at startup and lubricate better and then once the vehicle warms up turn into a "thicker" 40w oil?

I just replaced all my lifters due to one being flat. I "broke in" the new lifters using 10w-30 and the thing was as quiet as can be sitting idling at operating temp and through out the break in. I dropped the oil and went to a synthetic 10w-30 for daily use after the break in and it clatters like crazy. Go figure